In the grid-connection system of permanent magnet direct-drive wind power generation, the DC bus voltage of the inverter is usually used as the voltage outer loop control target. Due to the existence of measurement errors and noise , the bus voltage filtered by the inertial link is often used as feedback. In the traditional voltage outer loop active disturbance rejection control system, because the influence of the filter is not considered , there is a phase difference in amplitude between the control feedback and the actual system output, which affects the system control performance. The signal filtered through the first-order inertial link is expanded into a new state variable , and the improved linear expansion state observer estimates and feedbacks the bus voltage before filtering, which can eliminate the bus voltage ripple caused by the inertial link. The simulation results show that the system has strong anti- interference performance under the condition of observing noise disturbance and grid -side voltage fault.